1. Field of the Invention
This invention relates to a having a twin-cylinder type rotary compressor.
2. Description of the Related Art
There have been known air conditioners, each of which comprises a twin-cylinder type rotary compressor having a motor and two compression pumps and a cooling cycle system including a condenser, a decompressor and an evaporator all connected to the rotary compressor. One type of such air conditioners is disclosed in Japanese Laid-open Patent Application Hei 2-193089.
Each of the two compression motors of the twin-cylinder type rotary compressor in the disclosed air conditioner is set such that it has the cylinder operation volume half as much as that of the compressor pump of a single-cylinder type rotary compressor having a single compression pump.
To each compression pump is connected an electromagnetic two-way valve which effects the switching between the operation of a single compression pump and the parallel operation of two compression pumps. The motor of the rotary compressor is provided with an inverter circuit, the output frequencies of which are controlled according to the loads of the air conditioner. More specifically, the output frequencies are controlled such that both compressor pumps are operated in a parallel manner at a high load, and only a single compressor pump is operated at a low load.
The parallel operation of compression pumps provides a higher operation efficiency of the rotary compressor (the operation efficiency of the rotary compressor being hereinafter referred to as the n"COP") at an output higher than a common value at which the output curves of the parallel-operated compressor pumps and the singly operated compressor pump meet, while the operation of a single compression pump exhibits a higher COP at a lower output than the common output value. This characteristic of the rotary compressor improves the energy consumption efficiency of the air conditioner (hereinafter referred to as the "EER").
In the conventional air conditioner, the switching between the parallel operation and the single operation is carried out by assuming that the common output value of the output of the parallel-operated compressor pumps and the singly operated compressor pump or their COPs are always constant regardless of the loads applied to the air conditioner. However, the common values of the parallel-operated COP and the singly operated COP vary due to the loads applied to the air conditioner, making it unable to always obtain the optimum COP and hindering the EER from being fully improved.